The police officer suspended following the death of Ian Tomlinson during G20 protests has been questioned on suspicion of manslaughter, the Independent Police Complaints Commission said today.
He was questioned after a second post-mortem examination found Mr Tomlinson died from an "abdominal haemorrhage" and not a heart attack.
A spokesman for the Independent Police Complaints Commission (IPCC) said: "Following the initial results of the second post-mortem, a Metropolitan Police officer has been interviewed under caution for the offence of manslaughter as part of an ongoing inquiry into the death of Ian Tomlinson."
Pathologist Dr Nat Cary, who carried out a second post-mortem at the request of the IPCC and Mr Tomlinson's family, rejected the conclusions of the first.
He accepted that while there was evidence that Mr Tomlinson suffered hardening of the arteries in his heart, it was not serious enough to kill him.
In a statement, a spokesman for City of London Coroner's Court said: "Dr Cary's opinion is that the cause of death was abdominal haemorrhage. The cause of the haemorrhage remains to be ascertained.
"Dr Cary accepts that there is evidence of coronary atherosclerosis but states that in his opinion its nature and extent is unlikely to have contributed to the cause of death."
The first post-mortem, carried out by Dr Freddy Patel, concluded that Mr Tomlinson died from coronary artery disease.
The statement said: "Dr F Patel made a number of findings of fact including descriptions of a number of injuries and of diseased organs including the heart and liver.
"He found a substantial amount of blood in the abdominal cavity. His provisional interpretation of his findings was that the cause of death was coronary artery disease.
"The opinions of both consultant pathologists are provisional and both agree that their final opinions must await the outcome of further investigations and tests. These are likely to take some time."
A spokesman for the Metropolitan Police said the force was cooperating fully with the IPCC and would "proactively" give it any relevant information.
He said: "The Metropolitan Police Service wishes to reiterate its sincere regret in relation to the death of Ian Tomlinson.
"Our thoughts are with his family, and all those affected by this tragedy.
"As an independent investigation by the Independent Police Complaints Commission into the circumstances surrounding police contact with Mr Tomlinson is ongoing, we are unable to comment specifically on the findings of the post-mortem.
"We continue to cooperate fully with the IPCC and proactively provide any information that may assist them. We await the findings of the investigation."
Lindsey German, convener of the Stop The War Coalition, said: "This news is very disturbing and emphasises the need for a full and open public inquiry into recent aggressive policing of legal protests."
Mr Tomlinson's son Paul King said: "First we were told that there had been no contact with the police, then we were told that he died of a heart attack. Now we know that he was violently assaulted by a police officer and died from internal bleeding.
"As time goes on, we hope that the full truth about how Ian died will be made known."
The family's solicitor Jules Carey said they had been aware of the results of the second post-mortem for a week.
They had held the information back because the IPCC initially opposed its publication, fearing it would prejudice its investigation, he said.
He said the family hoped there would be a "prompt referral" to the Crown Prosecution Service for charge.
He said: "The video footage of the unprovoked and vicious assault on Ian by the police officer would easily justify charges of assault being brought against the officer.
"The findings of Dr Nat Cary significantly increase the likelihood that the officer will now face the more serious charge of manslaughter.
"The family have been aware of the findings of the second pathology report for a week and have had to endure the holding back of this information despite continuing reports in the press that Ian died of a heart attack.
"The IPCC opposed the disclosure of Dr Cary's findings until they satisfied themselves that it would not prejudice their investigation of the officer.
"It is of some comfort to the family that the record is now being put straight, but they hope that the IPCC investigation will be expedited and thorough, and that there will be a prompt referral to the CPS for charge."
London mayor Boris Johnson said: "People in London and across the country have been deeply disturbed by the death of Ian Tomlinson.
"Our first thoughts must be with his family as they mourn his loss. No one can doubt that the Metropolitan Police faced a huge challenge in securing the G20 summit.
"The police do an excellent job of making safe the 4,500 events and demonstrations that take place in London every year.
"But there must now be a fast and transparent conclusion to the IPCC investigation, with the full and urgent co-operation of all involved.
"Sir Paul Stephenson has rightly called in the IPCC and asked the HMIC (Inspectorate of Constabulary) to review the policing of demonstrations of this kind.
"It is vital that everyone takes care not to prejudice either the ongoing IPCC investigation or indeed any future criminal proceedings that may arise.
"The Metropolitan Police receive and deserve the overwhelming support of the people of London, but the family of Ian Tomlinson need answers, and so do Londoners."
Shami Chakrabarti, director of Liberty, said: "This new evidence will only add to the distress of Mr Tomlinson's family and further demonstrates how citizens and journalists have pressured the authorities into action.
"It increases the importance of the IPCC investigation and the Met Commissioner's broader review of policing tactics. We all owe it to Mr Tomlinson and his loved ones to ensure that this tragic death was not in vain."
In the immediate aftermath of Mr Tomlinson's collapse, Scotland Yard said officers trying to help him were pelted with plastic bottles and other missiles thrown by protesters.
Video footage then emerged of an officer striking the 47-year-old newspaper salesman with his baton before shoving him violently to the ground.
An amateur cameraman caught the policeman's actions on camera as thousands of protesters converged outside the Bank of England.
The officer, a member of the Territorial Support Group, was suspended last Friday at the request of the IPCC.
Liberal Democrat home affairs spokesman Chris Huhne said: "This is an alarming finding. It suggests that Mr Tomlinson's treatment by the police officer caught on video may have been the final contributing factor in his death.
"These findings put further pressure on the IPCC to investigate this matter with all urgency."